In recent years, a special awareness or interest has been placed on the quality of the environment. This has been due, in a large part, to the realization that many substances, both old and newly developed, can lead to present and future detrimental effects. Chemicals and toxic materials which adversely affect the air, earth or water present a serious concern as to the storing of such substances, and the subsequent environmental cleanup in the event of contamination of the environment by the inadvertent release of such materials.
One environmental concern that has prompted recent investigation and remedial action is that caused by petrochemical spills and leakages into the earth due to defective containers or pipelines. The oil, gas or fuel which contaminates the soil, if not checked, can pollute the soil as well as contaminate groundwater supplies and aquifers. One technique which has been employed effectively to decontaminate soil is to excavate the contaminated area and to process the soil through a kiln at an elevated temperature. While such a technique is effective, it is apparent that the time, cost and labor is appreciable and not cost effective for large contaminated areas.
The in situ treatment of contaminated soil has been carried out by use of neutralizing chemicals and solvents, as well as nutrients and microorganisms to promote in situ biodegradation of the contaminants. In addition, in situ soil flushing has been carried out by injecting solvents or surfactants into the soil to enhance the contaminant solubility. This technique involves the drilling of an extraction well in the contaminated soil zone, the drilling of reinjection wells upgradient of the contaminant area, and the construction of a waste water treatment system. Subsequent to the soil treatment, the groundwater is reinjected upgradient of the extraction well, which then leaches through the contaminated soil. The leachate is then collected, treated and reinjected back into the system, creating a closed loop system.
Yet another in situ treatment of contaminated soil involves a process in which production wells are drilled through the contaminated soil zone to a depth just above the water table. Monitoring wells are drilled around the production wells to monitor pressure gradients. A vacuum is then applied to the production wells. Because of the horizontal pressure gradient created in the soil zone by the vacuum pumps, volatiles in the soil percolate and diffuse through the air spaces between the soil particles to the production well. The vacuum established in the soil continuously draws volatile organic compounds and contaminated air from the soil pores, and draws free air through the soil surface down into the soil. The volatiles removed from the monitoring wells are then processed through a liquid-vapor separator. This procedure applies no heat input and is limited in both the rate of contaminant removal and the types of contaminants which can be vaporized.
In another variation of the foregoing technique, the treatment system includes injection wells for injecting steam, hot air and liquid chemicals into the churned soil. Extraction wells operating in a partial vacuum environment provide a horizontal pressure gradient in the soil. The mixture heats the soil and raises the temperature of the chemicals, eventually causing them to evaporate. The evaporated chemicals are drawn horizontally to the extraction wells and piped to a processing system that cools the chemical vapors for conversion into a liquid. The liquid chemicals are then further processed by an incinerator to detoxify the contaminants. One disadvantage of this technique is that the steam is prone to condense in the soil and form a liquid barrier to the further movement of contaminants to the extraction wells. Another disadvantage is that the soil temperature cannot be raised substantially above 212.degree. F. to remove less volatile contaminants
In U.S. Pat. No. 4,670,634, there is disclosed a technique for decontaminating soil by the use of radio frequency energy to heat the soil. Electrodes located over the surface of the decontaminated area radiate RF energy into the soil and heat the soil to the extent that gases and vapors are produced The rising gases and vapors are collected by a vapor barrier which operates under a slight vacuum. While the system appears to be effective, the energy requirements are substantial and costly, and the depth of the heat penetration into the soil is limited.
In those in situ decontamination systems where the soil is heated to either vaporize or oxidize the contaminants, there is the recurring problem of how to deliver the energy to the contaminated zone in the most efficient manner. As noted above, in heating the soil with RF energy, such a technique is costly and time consuming. Experimental in situ decontamination efforts have been carried out in which hot gases generated by surface heaters are carried by ducts and forced into injection wells. Since there is a limit to which air can be heated, conveyed and injected, there is a corresponding limitation to which the subsurface soil contaminants can be heated. As a result, soil contaminants characterized by low volatilities, such as polychlorinated biphenyls (PCBs), are difficult to remove from the soil as they require substantially high soil temperatures.
While many of the foregoing techniques are somewhat effective in providing in situ decontamination of the soil, many of the shortcomings attendant with such techniques are that the processes incur high operating expenses, require expensive equipment or chemicals, are limited in the rate at which energy can be introduced into the soil and as a final result are not overall extremely effective in reducing contaminants to a very low level.
From the foregoing, it can be seen that a need exists for a technique for the improved in situ removal of contaminants from a material. A further need exists for an efficient method and apparatus for the in situ decontamination of soil to a very low level, and which reduces the possibility of releasing or expelling such contaminants into the air. Another need exists for apparatus for controlling the injection of a heated gas into contaminated soil such that vapor condensation is minimized and permeability of the material to the heated gas is enhanced. Another need exists for the enhanced removal of contaminants by the use of a vertical pressure gradient to thereby eliminate the need for monitoring or extraction wells. Yet another need exists for an efficient utilization of energy, in which combustion gases, which used in heating the injection gas, are also injected into the contaminated soil to raise the temperature so that volatile and less volatile contaminants can be vaporized. Yet another need exists for a technique to deliver high temperature gases to subsurface formations to achieve oxidation of low volatile contaminants.